Grease Pencil - broken lines that should be smooth
I believe your problem is related to a GPU / Driver issue.  I am seeing the same thing you are seeing on a Mac with an ATI graphics card.  If I save my blend file and open it on a different Mac with an Nvidia graphics card the lines are no longer jagged / pixelated.  Check out this link for more information:  Weird artifacts with Grease Pencil V2

I have not yet found a work-around for this problem, other than to render the final image/sequence on a computer with an Nvidia card.